The present invention relates to masking part of a substrate's surface to facilitate selectively coating desired portions of the substrate.
Whilst the invention will be described with particular reference to masking portions of glazings or glass sheets it may also be used with other substrates.
In a coated car windscreen, it may be desired to provide a non-coated band around the periphery of the windscreen to reduce the likelihood of corrosion of the coating by ingress of moisture. Such a non-coated band may also provide electrical isolation between the edge of the windscreen and the coating layer if the windscreen's coating layer is electrically heatable and provide a portion at which electrical connectors may be arranged. A non-coated data transmission window may also be provided in the coated glazing, for example for the passage of infra-red or other data signals. Selectively coating desired areas of the glazing may also facilitate incorporation of heating systems, antennas and sensors.
U.S. Pat. No. 5,492,750 describes the use of reusable rigid, metal masks positioned over a glass substrate prior to depositing a coating layer so as to provide non-coated portions. Such masks are inconvenient to manipulate, require regular cleaning and are prone to produce an undesirable ghosting effect rather than a clean demarcation between coated and non-coated portions. A separate set of masks comprising a number of individual masks for each pattern of coating to be deposited is required for industrial production.
U.S. Pat. No. 5,713,986 describes the use of a water soluble resist material which is applied in a predetermined pattern over a substrate prior to the deposition of a coating layer. This resist material and the overlaying material are dissolved away so as to provide non-coated portions. The dissolution of the resist material requires applying water at high temperature which may be costly when used in a large scale process.
The resist material includes substances such as a water soluble film former, a surfactant, a rinsing agent and one or more solvents. The total weight percentage of the film-former, the surfactant and the rinsing agent is generally kept low, thereby providing a resist material which has a substantially low viscosity. When using screen-printing to apply the resist material, it is necessary to add a film thickener to increase the viscosity of the resist material. These film-thickeners may be insoluble in water at high temperature and thus difficult to remove from the substrate. They may react with the resist material and for example may clog the screen used for the printing. They may also have a limited shelf life which requires applying the masking agent within 24 hours after mixing the materials.
All these features cause handling and cost problems when using such a resist material in a large scale process for the manufacture of coated substrates.